The effects of the regulatory proteins skeletal muscle tropomyosin and gizzard tropomyosoin on the actin activated myosin subfragment-one (S-1) ATPase were compared at low ratios of S-1 to actin over a range of ionic strength. The effect of gizzard tropomyosin on the ATPase activity has a pronounced dependence on ionic strength. At low (20 mM) ionic strength gizzard tropomyosin inhibits the acto-S-1 ATPase while at high (120 mM) ionic strength it increases the ATPase. Over the same range of ionic strength, skeletal muscle tropomyoxin has a constant inhibitory effect. The effects of these two tropomyosins on acto-S-1 binding in the presence of AMP-PNP were also compared at high ionic strength. Not only were their effects on the ATPase quite different at this ionic strength, but their effects on binding were also different. While skeletal muscle tropomyosin confers cooperativity on the to acto-S-1 binding, gizzard tropomyosin has no measurable cooperative effect on this binding.